1. Field of the Invention
The present invention relates to a developing device, and an image forming apparatus.
2. Description of the Related Art
In an electrophotographic image forming apparatus such as a copier, a laser printer, and a facsimile machine, a process is performed of developing the electrostatic latent image formed on the latent image carrier (photosensitive drum) with the toner supplied from the developing device to form a toner image, transferring the toner image onto transfer sheet, and fixing the toner image.
Many conventional image forming apparatus using a dry developer including a toner and a carrier employ a developing device having a developer carrier (developing roller) that visualizes electrostatic latent images on a surface of the latent image carrier.
In the developing device, the toner is consumed by a developing operation, while the carrier is not consumed and remains in the developing device. Therefore, in the carrier stirred with the toner in the developing device, peeling of a resin coat layer on a carrier surface and adhesion of the toner onto the carrier surface occur, as stirring frequency increases. These phenomena cause contamination and progress of deterioration, thereby decreasing charging performance of the carrier.
Because the charging performance of the carrier as the developer gradually decreases, toners that do not hold an electric charge for charging is generated, and an image quality defect such as background stain of a print occurs, which is a phenomenon in which the toner adheres on a non-image portion on the photosensitive drum at the time of development, thereby causing considerable degradation of image quality.
To prevent the image quality defect such as the background stain resulting from deterioration of the carrier, the deteriorated developer in a developer container needs to be replaced regularly, and considerable maintenance labor has been consumed to replace the developer.
For example, to eliminate the need to replace developer, Japanese Patent Application Publication No. H2-21591 has proposed a developing device that replenishes a developer container with a small amount of developer including a mixture of carrier and toner into at the time of a normal operation for adding and replenishing the toner consumed due to a developing operation, and discharges deteriorated developer having deteriorated charging performance in a small amount from the developer container, thereby suppressing a decrease in the charging performance. This type of technique has been conventionally known as a “trickle developing method”.
According to the trickle developing method, the volume of a developer in a developer container increases due to replenishment of new carrier together with new toner into the developer container. However, the surplus carrier is allowed to overflow and discharged from a developer discharge port provided on a wall of the developer container, and collected in a developer collecting container.
By repeating replenishment and discharge of the carrier and the deteriorated developer, the developer contaminated and deteriorating in the developer container is replaced by newly supplied toner and carrier. Thus, the charging performance of the developer and the image quality are maintained.
In Japanese Patent Application Laid-open No. 2005-99134, a transfer member that stirs and transfers a developer including a toner and a carrier is formed of a first transfer unit having greater carrier capability and a second transfer unit having lesser carrier capability, and a discharge port is provided opposite to the second transfer unit. This configuration is for solving a problem such that a discharge amount of the surplus developer varies in a situation where vibration and impact are likely to occur in the developing device, which are generated at the time of rotating a developing unit in a revolving-type color developing device in which respective developing units storing each color toner rotate and move to a developing position, or at the time of supplying a recording sheet to a sheet feeder. Because a height of an upper surface of the developer largely varies vertically near a member such as a screw that stirs and transfers the developer, the upper surface position of the developer cannot be stably maintained relative to the discharge port, whose height is fixed. However, near the second transfer unit having lesser transfer capability, the height of the developer does not largely vary. Accordingly, the discharge port is set at the position, so that the developer in the developing device is maintained stably to stabilize the image quality.
In this case, however, because the developer discharge port provided on the wall of the developer container is fixedly arranged, if the developer adheres to the inside of the discharge port, an opening area becomes narrower, and the developer cannot be allowed to overflow and discharged stably from the developer container.
In Japanese Patent Application Laid-open No. H8-22190, to solve a problem such that when an opening for discharging a surplus developer is located on a wall of a developer container, the developer and the toner are accumulated in and adheres to a discharge port, whose position is fixed, to narrow the opening area, and the developer cannot be discharged from the developer container stably, and sequential replacement of the developer cannot be appropriately performed, an overflow unit is provided on the developer container side prior to the discharge port, so that the overflow unit blocks the developer transferred by the stirring unit to allow the developer to overflow in a predetermined amount from above the stirring unit is provided. The developer discharged from the overflow unit can be discharged by a transfer unit exclusive for discharge, and the discharge port can be arranged not only a side wall but also on the bottom of the developer container. Accordingly, the developer can be discharged without blocking the discharge port, and the developer amount discharged from the discharge port can be easily adjusted, thereby enabling appropriate sequential replacement of the developer.
However, it is configured such that the developer directed from a collection port to the discharge port is transferred in a backward direction flowing to the collection port, that is, to the developer container. Therefore, the developer is present in a dense state from the collection port of a return screw blade to the vicinity of the discharge port, which causes a problem such that rotating torque of the screw blade considerably increases. Although it is aimed to suppress the discharge amount by the return screw blade, the increase in torque accompanying this generates heat in the developer and causes deterioration of the carrier due to a thermal stress, thereby causing a decrease in stability of the image quality.